Complete replication: It implies maintaining of a whole copy of the database at every site. Thus, the reliability & availability and performance for query response are maximized. Though, storage costs, communication costs, and updates are costly. To overcome some of these troubles relation, snapshots are sometimes used. A snapshot is definite as the copy of the data at a given time. These copies are updated periodically, like, hourly or weekly. Therefore, snapshots may not always be up to date.